The preparation of lithographic printing plates is well-known in the art. The field of photolithography is discussed in "Light Sensitive Systems", by Jaromir Kosar, John Wiley & Sons, Inc., New York, 1965 (particularly in Chapter 3, Section 3.7) which discusses surface plates, deep etch plates, bi-metallic and tri-metallic plates, and plastic and paper lithoplates. Other sections of this book discuss in great detail the chemistry of photosensitive systems and, to the extent that they refer to photosensitive coatings and their application to a support, are also pertinent to the present invention.
Initially, lithographic printing was done with stone, but subsequently a great variety of materials have been used for supports. Thick zinc sheets have been popular which, because of their high cost, were reused many times after printing. For reuse the image was stripped and the surface of the plate cleaned and regrained laboriously. While zinc is still in use, aluminum is used in most plates and the trend has been to thinner sheets which are used but once and then discarded to realize savings in labor costs. Bi-metal and tri-metal plates are also used. Bi-metal plates are prepared by laminating or electroplating copper upon either stainless steel or aluminum of sufficient thickness to act as a practical support. The copper surface is coated with a photosensitive material which functions as a photoresist. After imaging and developing, the unprotected copper is etched away down to the level of the stainless steel or aluminum support. The photoresist is next stripped away, the copper is activated to make it oleophilic and the support metal is made hydrophilic by treatment with a gum solution, whereupon a printing plate ready for the press has been prepared. Such plates are known to have great run lengths.
Similarly, tri-metal plates are used, in which chromium is clad upon copper which, in turn, is clad upon a support metal. In this case, the support metal functions in a support capacity only and the image is in the copper (oleophilic) and the non-image is chromium (hydrophilic).
Tri-metallic and bi-metallic plates, as can be seen from the recitation of the necessary steps of preparation, are, despite their virtues, time consuming in preparation.
Newer developments have been in the use of aluminum sheet which is presensitized at the factory, the latest types of which are known as subtractive plates wherein a photosensitive coating combines sensitizing agent, pigment or dye with resin. After exposure and simple development, the resulting image has good thickness, a visible image and great resistance to press wear. The operations required to make such a plate after the exposure are no more than two (development followed by finishing) or more recently, only one, when a combined developer/finisher is used. Such plates offer high run lengths and great convenience to the user.
All of the aluminum and zinc plates previously described suffer from the disadvantage that, to be affixed satisfactorily to the plate cylinder of the printing press, they must be bent at either end and inserted into slots and carefully clamped on the cylinder. The plates representing the different colors in multi-color printing are particularly problematic since they must be carefully aligned to place them into register. The act of bending and clamping often causes plate failure due to metal fatigue, usually during longer press runs.
Speed in placing and removing plates on plate cylinders is particularly desirable in newspaper publishing, where operating speed is required, and for this reason letterpress plates on magnetic steel supports have been developed which adhere firmly to plate cylinders into which strong permanent magnets have been embedded. These are, however, letterpress plates. Some of the bi- and tri-metallic plates described above could be used on magnetic cylinders to gain the advantages of speed in placement and removability, if the stainless steel used could be of the magnetic variety. However, bi- and tri-metallic plates, as already described, suffer from the disadvantage of initial cost and further cost due to the numerous process steps required and additional time required for their preparation.
Entirely apart from their incapability of use in magnetic cylinder applications, the popular and convenient aluminum based plates suffer the further disadvantage of cost in manufacture because of the base metal employed, namely aluminum. Aluminum, as is well-known in its refinement from its ores, requires more energy than any other important metal and certainly much more so than steel so that this is a component of its cost which cannot be overcome unless someone is successful in extracting aluminum metal by non-electrolytic techniques. A search of the literature reveals no such prospects of accomplishing this at the present time.
Another disadvantage of aluminum, aside from cost, is its softness and ductility. Unless prohibitive thicknesses are used, great care must be taken in handling aluminum plates to prevent creases or dimples, which cannot be flattened once creased and which make a fully processed plate useless.
The desirability of using a steel plate, because of its greater strength than aluminum, its lower cost than aluminum and to take advantage of its utility as a magnetic material, has long been known. However, it has not been possible to employ steel for this purpose because of the speed with which steel corrodes in air and in contact with press chemicals and, even more seriously, because coatings of photosensitive materials, primarily diazos, upon steel suffer rapidly from dark reaction which makes them worthless as presensitized plates because of lack of storage stability.
Plain steel sheet has been considered for plates because of availability and lower cost, but has found no place in the art because of its severe corrosion problems.
Corrosion is a problem in steel for any use. However, in lithographic printing plates, it cannot be tolerated at all. There is voluminous literature on the protection of steel from corrosion by phosphating, passivating, coating and painting, but such treatments do not result in a suitable carrier for presensitized printing plates. In fact, most known treatments render the steel totally unsuitable as a lithographic carrier.
It is the object of this invention to demonstrate how, quite unexpectedly, the shortcomings mentioned above can be overcome. A more specific object is to teach how to prepare a steel carrier for use as a presensitized plate, which, in turn, can then be used on magnetic plate cylinders. It is another object of this invention to thereby prepare a presensitized metal plate which is inherently cheaper than the popular aluminum-based presensitized plates and for that matter any of the bi-metallic and tri-metallic plates known today. It is a further object of the invention to prepare a plate which is more durable and crease resistant than an aluminum plate. Yet another object of this invention is to provide a plate which will have good dimensional stability, which is not obtainable in any plastic-or paper-based plate.